For sonar systems, it is extremely important to grasp an acoustic wave propagation direction undersea. However, estimation of an acoustic wave propagation direction undersea is difficult, because the refractive index of acoustic wave varies depending on differences in temperature distribution, density distribution and salinity distribution.
In general, it is widely performed to measure temperature distribution of seawater in the vertical direction in advance of or simultaneously with undersea operation of a sonar system, or measure also salinity distribution simultaneously with the temperature distribution, and accordingly to estimate distribution of the refractive index of acoustic wave from the measured temperature distribution only or from both of the measured temperature and salinity distributions.
For example, as described in Non-patent Literature 1, a device for measuring only temperature distribution is generally referred to as XBT, and a device for measuring both temperature distribution and salinity distribution as XCTD. It is needless to say that, even without actually performing such measurement, estimation of sound velocity distribution from an average of previous measurement results is also widely performed,
Also widely used is a method referred to as ocean acoustic tomography which, as Non-patent Literature 2 describes its outline, directly measures sound velocity distribution by installing a plurality of transmitter/receivers distributively in a target area of the sea and operating them to transmit and receive acoustic waves to and from each other.